The present invention relates to methods and apparatus for logging earth formations penetrated by a borehole, and more particularly to methods and apparatus for directly determining formation characteristics by analysis of gamma rays from naturally occuring formation radiation. The invention is particularly directed to techniques for the quantitative identification, measurement, and evaluation of the abundance of uranium (U) in uranium ore zones using such borehole radiation measurements.
In recent years gamma ray spectroscopy of earth formations in the vicinity of boreholes penetrating those formations has been made practical by the development of highly stable scintillation detectors which may be lowered into the boreholes and produce a pulse height spectrum proportional to the gamma ray energy spectrum of gamma rays impinging upon the scintillation crystal. As a result, a variety of borehole measurement techniques is now available for use in ore body evaluation. Current quantitative uranium ore identification and measurement techniques rely on either the detection of induced fission neutrons, integral measurements of gamma ray intensity, or analysis of gamma ray spectra to identify specific gamma rays from uranium or its daughters. All of these techniques have inherent limitations or drawbacks. The measurement of fission neutrons requires a pulsed neutron source and rather sophisticated electronic gear, and is affected by the neutron moderation and absorption properties of both the borehole and the medium containing the ore. The gross gamma count method and the spectral uranium identification method can be adversely affected by natural radioactive disequilibrium, and require accurate knowledge of probe efficiency, borehole size and fluid (and casing or cement, if present), tool position in the borehole, and dead time loss of the counting system in high concentration ore bodies.
There is therefore a need for an improved method and apparatus for quantitatively evaluating the abundance of uranium in ore zones in the vicinity of earth formations penetrated by a borehole. Such a method and apparatus should be virtually independent of the specific elements present in the borehole, be unaffected by the tool and borehole geometries, and not be strongly related to gamma ray energy.